380 PHYSICAL CONDITIONS IN GENESIS OF SPECIES. 



In the gray fox, whose habitat extends from Pennsylvania southward 

 to Yucatan, the average length of the skull decreases from about 5 

 inches in Pennsylvania to considerably less than 4 in Central 

 America — a difference equal to about 30 per cent of the mean size for 

 the species. 



The Felida^, unlike the Canida^, reach their greatest development, 

 as respects both the numl)er and the size of the species, in the inter- 

 tropical regions. This family has but a single typical representa- 

 tive — the panther {Felis concolor) — north of Mexico, and this ranges 

 only to about the northern boundary of the United States. The other 

 North American representatives of the family are the lynxes, Avhich, 

 in some of their varieties, range from Alaska to Mexico. They form, 

 however, the most northern as well as the most specialized or " aber- 

 rant " type of the family. While they vary greatly in color as w^ell 

 as in the length and texture of the pelage at different localities, they 

 afford a most remarkable exception to all laws of variation in size 

 with locality; for a large series of skulls, representing localities as 

 widely separated as Louisiana, northern Mexico, and California on 

 the one hand and Alaska and the Mackenzie River district on the 

 other, as well as various intermediate localities, reveals no appreciable 

 difference in size throughout this wide area. The true cats, however, 

 as the panther and the ocelots, are found to greatly increase in size 

 southward or toward the metropolis of the family. The panther 

 ranges from the Northern States southward over most of South 

 America. Skulls from the Adirondack region of New York have an 

 average length of about 7\ inches, the length increasing to 8f in 

 Louisiana and Texas, from beyond which points there is lack of data. 

 The ocelot {Felis pardalis) finds its northern limit near the Rio 

 Grande of Texas, and ranges thence southward far into South 

 America. The average size of Costa Rican examples is about one- 

 fifth greater than that of specimens froui the Rio (jrande. 



The Procyonidfx? are chiefly represented in tropical America, a 

 single species— the common raccoon {Procyon Jotor) — being found in 

 the United States, and thence northward to Alaska [=British Co- 

 lumbia]. Here again the increase in size is southward or toward 

 the metropolis of the family — Pennsylvania specimens averaging 

 about one-tenth smaller than Costa Rican examples. 



The common otter {Lvtra canadensis) affords another example of 

 increase in size southward among our Carnivora, although belonging 

 to a family essentially northern in its distribution. The otters, how- 

 ever, form a distinct subfamily, Avliich attains its greatest number of 

 species in the warmer regions of the earth, and hence offers not an 

 exception to but a confirmation of the laAv of increase toward the 

 center of distribution of the group to which it belongs. 



